As a mechanism for converting rotational motion into linear motion, for example, a conversion mechanism disclosed in Patent Document 1 has been proposed. The conversion mechanism includes a annular shaft, which has a space extending in the axial direction therein, a sun shaft, which is arranged inside the annular shaft, and planetary shafts, which are arranged around the sun shaft. Also, externally thread portions formed on the outer circumference of the planetary shafts are engaged with an internally thread portion formed on the inner circumference of the annular shaft, and an externally threaded portion formed on the outer circumference of the sun shaft. Thus, force is transmitted among these components. Sun-and-planet motion of the planetary shafts that is obtained when the annular shaft is rotated causes the sun shaft to move linearly along the axial direction of the annular shaft. That is, the conversion mechanism converts the rotational motion input to the annular shaft into the linear motion of the sun shaft.
In the above mentioned conversion mechanism, two gear mechanisms are provided such that force is transmitted by the engagement of the gear mechanisms in addition to the engagement of the threaded portions between the annular shaft and the planetary shafts. That is, the above-mentioned conversion mechanism includes a gear mechanism which is configured by a first annular gear provided at one end of the annular shaft and a first planetary gear provided at one end of the planetary shaft so as to engage with the first annular gear, and a gear mechanism which is configured by a second annular gear provided at the other end of the annular shaft and a second planetary gear provided at the other end of the planetary shaft so as to engage with the second annular gear.
In the conversion mechanism of the Patent Document 1, when the rotational phase of the first annular gear differs from the rotational phase of the second annular shaft gear, the planetary shafts are arranged between the annular shaft and the sun shaft in an inclined state with respect to a reference position (position where the center lines of the planetary shafts are parallel to the center line of the sun shaft). Thus, the engagement of the threaded portions becomes uneven among the annular shaft, the planetary shafts, and the sun shaft. This increases local wear, thus reducing the conversion efficiency of rotational motion into linear motion. Such a problem occurs not only in the above conversion mechanism but in any conversion mechanism including gear mechanisms configured by the gears of the planetary shafts and the gear of at least one of the annular shaft and the sun shaft. Patent Document 1: International Publication WO2004/094870